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EPI – Echo Planar Imaging

EPI – Echo Planar Imaging. Joakim Rydell 2004-03-26. Echo Planar Imaging. Like ordinary MR, but faster Ordinary MR: approximately one minute EPI: less than one second The Nobel Prize in Medicine 2003 Lauterbur (MR) Mansfield (EPI). Repetition, k-space. Fourier transform of image

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EPI – Echo Planar Imaging

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  1. EPI – Echo Planar Imaging Joakim Rydell 2004-03-26

  2. Echo Planar Imaging • Like ordinary MR, but faster • Ordinary MR: approximately one minute • EPI: less than one second • The Nobel Prize in Medicine 2003 • Lauterbur (MR) • Mansfield (EPI)

  3. Repetition, k-space • Fourier transform of image • Traversed using magnetic field gradients

  4. Repetition, pulse sequence • Slice selection • Phase encoding • Frequency encoding

  5. Introducing EPI Ordinary MR • Acquire all rows in k-space with just one excitation EPI

  6. EPI – The Movie

  7. Going even faster – Partial Fourier • Real image  hermitian symmetric k-space • Acquire half of k-space, compute the rest • Not always as good as it seems...

  8. Partial Fourier, drawbacks • Phase errors • Slowly varying • Approximately 65 % of k-space required • SNR decrease • Full k-space acquisition gives sqrt(2) SNR improvement

  9. Why use ordinary MR? • Signal decay • Low SNR • N/2 ghosting • Geometric distortion • dB/dt

  10. Signal decay • Long readout period • Weak signal near end of acquisition • Blurring in phase-encoding direction

  11. Low SNR • Fast acquisition • Less averaging • Noisy images

  12. N/2 ghosting • Alternating acqusition directions • Shifted sampling positions • N/2 ghosting • Reference scans

  13. Geometric distortion • Low bandwidth/pixel • Pixels shifted in phase encoding direction • Field maps

  14. dB/dt • Fast gradient switching induces eddy currents, causing field inhomogenity • Rapid changes in the magnetic field can cause peripheral nerve stimulation

  15. k-space trajectories • Cartesian • Blurring • N/2 ghosting • High dB/dt • Spiral • Non-uniform sampling • Cartesian spiral

  16. Ramp sampling • Sampling on gradient ramps • Non-uniform sampling • In time • In k-space

  17. Multishot • Both EPI and ordinary imaging • Collect several rows per excitation • Alignment difficulties

  18. Applications • Cardiac imaging • fMRI • ...

  19. EVI – Echo Volumar Imaging • 3D EPI • Acquire a full volume with just one excitation • Not quite possible yet

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